ABSTRACT For the first time on a national level, we propose to utilize routinely-collected laboratory data to develop a cohort of pregnant women living with HIV in South Africa in a way that is uniquely robust to transfer across facilities. South Africa is home to more people living with HIV than any other country, and nearly one in three pregnant women attending antenatal care is living with HIV. Access to antiretroviral therapy (ART) has increased substantially since the start of the national ART program in 2004, with >95% ART coverage during pregnancy and delivery, and mother-to-child transmission of HIV greatly reduced. However, our work has demonstrated that women who initiate ART during pregnancy are at heightened risk of dropping out of care, particularly after delivery, leading to the potential for viral transmission, morbidity and mortality. It is difficult to evaluate the success of policies of expanded access to ART care, and assess continuity of care, due to the lack of a national longitudinal HIV care database. In addition, patient movement between unlinked facilities means that self-transfer often is misclassified as loss to care. To address this major shortcoming, our team at the Health Economics and Epidemiology Office (HE2RO) partnered with the South African National Health Laboratory Services in 2015 to use existing HIV-related laboratory data to create the South African National HIV Cohort. Now, through this five-year award, we propose to expand this successful project to use routinely- collected laboratory data to develop a cohort of pregnant women living with HIV in South Africa. We propose three specific aims. First, we will develop and validate a novel national cohort of pregnant women accessing HIV care. Using laboratory test data matched to facility type, we will identify entry to antenatal care to build the cohort, then describe key treatment milestones, including 1) engagement in antenatal care, 2) initiation of ART, 3) HIV viremia, and 4) continuity of HIV care in the postpartum period. Second, we will measure the effect of system-wide factors impacting continuity of care among pregnant women. We will assess policies of expanded treatment access on continuity of care using regression-discontinuity analyses. We then will assess mobility and its effect on continuity of care during and after pregnancy. Third, we will identify individual-level risk factors for loss from HIV care in order to develop targeted interventions to improve engagement in HIV care.